The invention relates to a method for testing a capacitive actuator or actuating element, particularly an actuating element for a fuel injection valve in an internal combustion engine, for correct operation.
DE 199 10 388, which is not a prior publication and has earlier priority, describes a method for testing a piezo-electric actuating element, particularly one for a fuel injection valve, in which the actuating element's open period, ascertained from the charging and discharge currents on the actuating element by comparison with threshold values, is compared with the duration of the control signal and this comparison is used to diagnose correct operation of the actuating element. This method can be applied when the charging period and discharging period are the same length, since otherwise the actuating element's open period is not equal to the duration of the control signal.
To open, by way of example, a fuel injection valve in an internal combustion engine, an electric charge needs to be applied to the actuating element and must be removed from the actuating element again in order to close the injection valve. For a constant fuel pressure, for example in a common rail fuel injection system, the injected quantity of fuel is primarily dependent on the length of the injection period.
When any capacitive actuating element is charged, a charging current flows into the actuating element; the latter is charged when the charging current becomes zero again. During the charging operation, the actuating element voltage drop across the actuating element rises to a particular value. In the charged state, no current flows, and the actuating element voltage remains approximately constant. During discharge, a discharge current flows out of the actuating element; the latter is discharged when the discharge current becomes zero again. During the discharging operation, the actuating element voltage which is on the actuating element falls to zero volts again.
DE 198 45 042 A1 discloses a method for diagnosing a capacitive actuator, where the actuator is supplied with a prescribable amount of energy, and incorrect operating states of the actuator are inferred by comparing measured values for the actuator current, actuator voltage or actuator charge with prescribed comparative values.
DE 199 44 734, which is not a prior publication and has earlier priority, describes a method for driving a capacitive actuating element with different charging and discharging periods. The shorter the charging and discharging operations are, the more noise intensive they become.
The control operations can be disrupted by internal or external influences such that the charge applied to the actuating element remains on the actuating element for longer than prescribed by the control signals output by an engine control system, and the fuel injection valve remains open for an undefined period, which results in too much fuel being injected.